Why do hearses have 4 doors?

So that you can access the casket at the front end of the car. This is used often for placement of flowers or a flag on the casket after it is placed in the car, but before it comes out of the car at the cemetery.

Also, hearses are built on a limousine chassis, so the door was already there, so it was easier just to leave it there, since it did provide some function.